1. Field of the Invention
The present invention relates to the field of dense wavelength division multiplexing (DWDM) optical telecommunication components, and more particularly, it relates to passive multiplexing or demultiplexing devices, which separate multiple channels of light co-propagating in one optical fiber into multiple individual fibers (or the reverse).
2. Description of Related Art
The amount of data that can be transferred using an optical fiber is based on the bandwidth that the fiber can support This value can be very large for modern fibers, leading to a theoretical limit of >10 Tbits/s for a single fiber cable link. However, in practice this bandwidth cannot be fully utilized, primarily due to electronic limitations in the generation and reception of laser beams modulated at this very fast rate. Wavelength division multiplexing (WDM) is way to utilize more of the available bandwidth in an optical fiber by sending multiple individual channels, with different wavelength, down a single fiber. The achievable data rate is now dependent on the number of channels multiplied by the data rate in each channel, which contains data at a rate able to be processed by modern electronics. WDM and Dense-WDM (DWDM) systems have been deployed extensively for long-haul telecommunications links and are now being installed in shorter-range links such as metropolitan areas.
One of the key components for DWDM systems is a device which allows multiple beams of light, each having a slightly different wavelength corresponding to a communication channel, which are co-propagating in a single-mode optical fiber, to be separated spatially so that the information in each channel can be processed. Also, the reverse of this is needed, whereby individual channels having different wavelengths are recombined from individual fiber optic cables into a single cable. The performance of these devices is determined by their size and stability, the losses that each output channel incurs, and the amount of stray light that leaks between channels (cross-talk).
Prior art exists for various technical solutions to this problem. Early solutions used a prism to provide the chromatic separation. For example, see U.S. Pat. No. 3,845,294, titled “Multiplexed Communications System”, issued Oct. 29, 1974. U.S. Pat. No. 3,863,063, titled “Optical Communications Systems” issued Jan. 28, 1975, also describes a pulse-shaping system that uses a diffraction grating to cause chromatic dispersion, but this was not a multiplexer device. A diffraction grating was used within a low loss multiplexing system in U.S. Pat. No. 4,111,524, titled “Wavelength Division Multiplexer”, issued Sep. 5, 1978).
Other methods for multiplexing include multiple thin-film interference filters (see e.g., U.S. Pat. No. 4,474,424, titled “Optical Multi/Demultiplexer Using Interference Filters” issued Oct. 2, 1984); fiber-based interferometers incorporating fiber Bragg grating filters (see e.g., U.S. Pat. No. 5,457,760 titled “Wavelength Division Optical Multiplexing Elements” issued Oct. 10, 1995); and arrayed waveguide gratings (see e.g., U.S. Pat. No. 5,913,000 titled “Method And Apparatus For Guiding Optical Signals” issued Jun. 15, 1999). There are many others examples of patented concepts in this area, where these basic system designs are modified to improve loss, package size, and cost. Relevant examples are: U.S. Pat. No. 5,526,155 titled “High-Density Optical Wavelength Division Multiplexing” issued Jun. 11, 1996; U.S. Pat. No. 5,355,237, titled “Wavelength-Division Multiplexed Optical Integrated Circuit With Vertical Diffraction Grating” issued Oct. 11, 1994 and U.S. Pat. No. 4,930,855, titled “Wavelength Multiplexing Of Lasers” issued Jun. 5, 1990.
It is desirable to provide control over the separation or combination properties of the individual wavelength channels of passive multiplexing or demultiplexing devices in the most recent types of systems (e.g., DWDM systems where there are >80 individual channels, separated by <50 GHz).